The present invention relates to an optical waveguide device which is less or scarcely dependent on the plane of polarization.
In the optical fiber communication, the light wavelength multiplex transmission technique is very important from the view point of achieving economy and flexibility of the optical communication system. On the other hand, optical coupling/branching devices are indispensable for the light wavelength multiplex transmission.
In conjunction with fabrication method of the optical coupling/branching device, there is noticed in recent years the trend of studying and developing a waveguide structure of the optical coupling/decoupling device, aiming for implementation in a one-chip monolithic structure and reduction of manufacturing cost. A typical example of such optical device, there can be mentioned a directional coupler type optical branching device disclosed in N. Takato et al's article entitled "Low-loss Directional Coupler Using High-silica Embedded Channel Waveguides" contained in "OEC '86 Technical Digest", A3-3 (July, 1986), pp. 22 and 23. In this known device, optical branching characteristics are obtained by making use of the fact that the coupling of two optical waveguides is dependent on wavelength. More specifically, light signals of wavelengths .lambda..sub.1 and .lambda..sub.2 inputted to a first port travel through a coupling region while reciprocating periodically between two cores, wherein the light signal of wavelength .lambda..sub.1 is led out through a third port with the light signal of wavelength .lambda..sub.2 being taken out from a fourth port. For the substrate of the optical device, a Si-substrate or SiO.sub.2 -substrate is usually employed. A low refractive index layer (having a refractive index n.sub.b) is formed of a SiO.sub.2 -film. The refractive index n.sub.c of the core has to be greater than the index n.sub.b of the low refractive index layer. For this reason, the SiO.sub.2 -film is ordinarily doped with refractive index controlling dopant such as TiO.sub.2, P.sub.2 O.sub.5, GeO.sub.2 or the like. A cladding layer is formed of a material having the same refractive index as that (n.sub.b) of the buffer layer (low refractive index layer) and is usually formed from SiO.sub.2.
In the structure of the optical waveguide device mentioned above, thermal expansion coefficients of the core, the low refractive index layer and the cladding layer differ from one another. FIG. 11A of the accompanying drawings shows relations between the contents of various dopants and the refractive indexes, and FIG. 11B shows relations between the content of various dopants and the thermal expansion coefficients. As will be seen in FIGS. 11A and 11B, when the dopant TiO.sub.2, GeO.sub.2, P.sub.2 O.sub.5 or other is contained in SiO.sub.2, the thermal expansion coefficient of the latter assumes correspondingly different values. Further, as the content of dopant increases, magnitude of change in the thermal expansion coefficient becomes more remarkable. As a consequence, thermal stress is produced between the core and the cladding, between the core and the low refractive index layer and additionally between the low refractive index layer and the substrate. The thermal stress in tern brings about anisotropy in the refractive indexes, resulting in occurrence of shift of wavelength which exerts adverse influence on the coupling efficiency between E.sup.11X -mode and E.sup.11Y -mode. The wavelength shift involves degradation in isolation in the optical waveguide coupling/branching device, leading to increasing of the cross-talk between the light signals of different wavelengths.